For numerous motor or sensory handicaps and pathologies, electrical stimulation of nerve structures has been proposed and even clinically validated. It involves, for example, high-frequency electrical stimulation for the treatment of Parkinson's disease, stimulation of the inner ear for the treatment of deafness, and, more recently, of the retina or the visual cortex for the treatment of blindness. However, a very large number of applications can be envisaged, in particular for sphincter control, the treatment of epilepsy and other neurological diseases.
To implement this type of stimulation, an implant is placed in contact with the nerve structure concerned. Such an implant has electrodes with which an electrical potential difference is applied, or a current is injected, so as to stimulate the nerve cells. A number of electrode configurations have been proposed in order to obtain effective stimulation of the targeted structure.
In the so-called monopolar configuration, current flows between a stimulation electrode and a remote return electrode (at an infinite distance). This monopolar configuration provides stimulation of poor spatial selectivity. However, the spatial selectivity of the electrical stimulation is a desired property in many applications. For example, if the implant has an array of microscopic units juxtaposed so as to perform electrical stimulations independent of one another, in particular to communicate a sharp image to neurons of the retina or the visual cortex, it is important to provide electrical stimulations that are well-localized at each of these units or pixels, with electrical leakage (cross-talk) as low as possible between adjacent pixels.
Bipolar configurations use a pair of electrodes for each area of the nerve structure to be stimulated, excited by positive and negative electrical potentials. The localization of the electrical stimulation is improved with respect to the monopolar configuration, but may still be insufficient for certain applications.
In practice, electrode configurations with a ground plane are preferable to bipolar electrodes since the return electrode is then common to all of the units of the implant, thus dividing the internal wiring of the system by two. In “Improved Focalization of Electrical Microstimulation Using Microelectrode Arrays: A Modeling Study” (PLoS ONE, www.plosone.org, Vol. 4 , no. 3 , e4828 , March 2009), S. Joucla and B. Yvert showed improved focusing of the microstimulation with a component of which the surface has a ground plane coplanar with stimulation electrodes.
In “Migration of retinal cells through a perforated membrane: implications for a high-resolution prosthesis” (Investigative Ophthalmology & Visual Science, September 2004 , Vol. 45 , no. 9 , pages 3266-3270), D. Palanker et al. studied the capacity of rat retinal cells to migrate thorough an electrically inert perforated membrane, and imagined an implant with a three-dimensional configuration with electrodes projecting over a membrane. However, such an implant appears to be difficult to produce in practice.
In general, it is desirable to produce localized stimulations capable of performing their stimulation role without damaging the tissues. Clinical studies have shown that the current intensities making it possible to obtain a response in the targeted neurons may exceed the safety thresholds for the tissues. Moreover, applications such as vision require the number of electrodes to be multiplied for the same total size of the implant, and therefore to increase their spatial resolution.
There is therefore a need to design electrode structures that make it possible to increase the focusing of the stimulations while limiting the amplitude of the currents generated.